High capacity fiber-optic communications systems are often used to provide highly reliable data communications between two points. Some fiber-optic communications systems are based on the SONET (Synchronous Optical NETwork) standard. The SONET standard includes a number of Optical Carrier (OC) specifications that define the data rate carried by a fiber-optic line. One example of data transmission levels of SONET systems is SONET OC-3, which defines specific data rate and multiplexing schemes for signals carried across the fiber-optic line. Some SONET systems communicate data by using a redundant pair of fiber-optic lines to connect two endpoints such that a fault or failure on one of the fiber-optic lines or its endpoint equipment is accommodated by use of the other redundant fiber-optic line. SONET systems generally carry multiplexed data that are organized as bearer channels. Examples of bearer channels include digitized voice channels used to carry telephone conversations. The multiplexed data carried over a SONET fiber-optic line are generally generated for transmission and processed upon reception by a bearer processor. The bearer processor accepts multiple streams of user data and generates the OC-3 data stream for transmission. The bearer processor also accepts a received OC-3 data stream, extracts the user data and delivers it to external interfaces for delivery to other user equipment.
The processing equipment located at each end of each single fiber-optic communications line within a redundant fiber-optic pair is generally located in one card cage. The fiber-optic lines generally terminate at a SONET line card that extracts a raw OC-3 data stream to be provided to a bearer processor within the same card cage. The SONET standard has been augmented by an Automatic Protection Switching (APS) mechanism to standardize processing to detect faults or failures and switch between fiber-optic lines of the redundant fiber-optic pairs. The SONET APS, however, does not support component fault management or failure protection processing across cages or for the bearer path beyond the SONET line card. Although the SONET APS provides availability enhancement by effectively using redundant fiber-optic lines to recover from some faults or failures, the availability of the overall communications system is still limited by the effective signal path being forced back to a bearer data path that is susceptible to a single point of failure inside the bearer processing equipment within each card cage.
Data communications transport components usually require availability on the order of six nines (i.e., an availability of 0.999999) rather than the five nines, (i.e., 0.99999) that is normally required for bearer processing network elements. The availability of that single card cage, due to anticipated failures of components within the cage, is often the “noise floor” limiting availability of a fiber-optic data communications system beyond five nines.
Therefore, an availability improvement to failure handling in redundant fiber-optic pair SONET communications system is needed.